Serosurvey in two rural areas evidences recent and previously undetected WNV and SLEV circulation in Costa Rica

West Nile virus (WNV) and Saint Louis encephalitis virus (SLEV) share similar virus transmission cycles that involve birds as amplifiers and mosquitoes as vectors. Mammals, including humans, are dead-end-hosts that may be asymptomatic or develop more severe symptoms. Costa Rica is a hyperendemic country for several flaviviruses such as Dengue (DENV) and Zika (ZIKV) and previous research showed limited and restricted SLEV and WNV circulation in horses, sloths, and monkeys. Nevertheless, actual seroprevalence and high transmission areas are not yet identified. To determine putative WNV and SLEV circulation, we sampled peri-domestic and domestic animals, humans, wild birds, and mosquitoes in rural households located in two DENV and ZIKV hyperendemic regions during the rainy and dry seasons of 2017-2018 and conducted PRNT assays for serology and RT-PCR for virus detection. At Cuajiniquil, serological evidence of WNV and SLEV was found in equines, humans, chickens, and wild birds. Also, 5 seroconversion events were recorded for WNV (2 equine), SLEV (1 human), and DENV-1 (2 humans). At Talamanca, a lack of WNV circulation was found, but evidence of SLEV circulation was recorded in equines, humans, and wild birds. No evidence of active viral infection was found in any mosquitoes or wild bird samples. This seroconversion evidence supports the active and recent circulation of SLEV and WNV in these two regions. This study provides clear-cut evidence of WNV and SLEV circulation and should be considered by the health and epidemiology authorities for future prevention and differential diagnostics.